The present invention relates to apparatus and to a method for metering liquids and, in particular, to dye-metering apparatus for metering liquid colouring agents for the preparation of varnishes and paints.
The term xe2x80x9cliquidxe2x80x9d as used in the present description is intended to include viscous and non-viscous liquids, dispersions and fluid suspensions.
In the field of machines for metering dyes for varnishes and paints, manually-operated, semi-automatic or automatic machines with pistons are widely known and are widespread. Machines with pistons comprise, for each colouring agent to be metered, a reservoir communicating with a cylinder having a sliding piston the operation of which brings about the delivery of the required metered quantity. Manually-operated machines are the most widespread owing to their relatively low costs, but they suffer from serious disadvantages in terms of accuracy in volumetric metering. In automatic piston machines, the piston of each cylinder is operated by a respective stepper motor; although automation improves ergonomics and ease of use, it gives rise to a substantial increase in both production and maintenance costs.
Automatic dye-metering machines which use a geared volumetric pump driven by a respective motor and a solenoid valve for the delivery of the liquid for each liquid reservoir and which enable the liquid to be recirculated to the reservoir are also known. The main disadvantage of this type of machine relates to the high costs of both production and maintenance, as well as to limitations in relation to the liquids which can be metered. The geared pump is in fact not suitable for metering dyes containing micaceous substances and cannot therefore be used in the preparation of varnishes for the automotive or xe2x80x9ccar refinishingxe2x80x9d field.
Within the field of automated machines, machines with sequential operation, in which the liquids to be metered are supplied to the collecting container in succession, and machines with simultaneous operation, in which the liquids for the formulation of the desired composition are supplied to the collecting container simultaneously, are available.
In general, in automatic machines, the major portion of the production cost results from the need to transfer the liquids from the supply reservoir to the collecting container at a constant flow-rate which can be controlled by a computer-controlled shut-off system. For this purpose, as has been seen, expensive components such as stepper motors, encoders, graduated pistons, geared pumps and pneumatic pumps are used. To reduce costs, apparatus currently available has a centralized control system for all of the metering circuits installed, but the products to be metered have extremely diverse characteristics and often require dedicated flow-rate and pressure arrangements. In particular, apparatus which is currently available on the market and uses geared pumps has high maintenance and spare-parts costs precisely for the part which controls the flow-rate and the movements of the fluids. This apparatus also requires frequent calibration due to wear of the system or to leakages in the circuit, with consequent loss of accuracy in the small dosages which are extremely important in colour production; moreover, the system used in these machines for achieving a constant flow-rate requires moderate fluctuations of electrical current (which are not always present, particularly in developing countries), requiring the addition of a UPS system with a consequent further increase in costs for the final user in these cases.
A further known problem in all of the apparatus which is currently available and which generally has product reservoirs exposed to the air, is the drying of the liquids along the wall of the reservoir as the liquid level gradually falls, as well as evaporation of the vehicles of the liquids. As well as causing considerable variations in the rheology of the product and varying its dyeing power (in the case of dyes), with a detrimental effect in colour reproduction, these two factors require constant maintenance of filters or other systems for intercepting hard particles contained in the liquids, which invalidate the precision of the circuit. As stated, a further limitation in systems which use geared pumps is that it is impossible to meter pearly or micaceous products since they are ground up by the pumps, altering the desired final result.
The object of the present invention is to provide apparatus for metering liquids which overcomes the above-mentioned disadvantages and which also has low production and maintenance costs.
For this purpose, the subject of the invention is apparatus for metering one or more liquids contained in respective reservoirs into one or more collecting containers, characterized in that it comprises, for each liquid:
a cylinder for the liquid to be metered, the cylinder having a movable piston member in contact with the surface of the liquid in said cylinder,
pressure means for exerting a constant force on the piston,
means for the transfer of the liquid from the cylinder to the collecting container, the transfer means comprising a duct having an inlet opening for the withdrawal of the liquid from the cylinder and a delivery opening, said transfer means being movable as a function of the level of the liquid in the cylinder and being arranged to keep the geometrical height difference between the liquid-withdrawal point and the delivery point proportional to the hydrostatic head of liquid above the withdrawal point as the level of liquid in the reservoir varies, so as to keep the delivery flow-rate constant, and
valve means associated with the transfer duct for intercepting and controlling the flow of liquid in the transfer duct.
In a preferred embodiment, the means for the transfer of the liquid from the cylinder to the collecting container comprise an inlet opening communicating with the cylinder for the withdrawal of the liquid and fixed to the piston, and a nozzle for delivery into the collecting container, the nozzle being connected rigidly to the movable piston so that the hydrostatic head of liquid above the inlet opening and the difference in level between the inlet opening and the delivery opening are kept constant as the level of liquid in the cylinder varies.
According to the invention, the cylinder can operate at atmospheric pressure or under pressure. In the atmospheric pressure version, which is usable particularly for nonviscous liquids which do not offer great flow resistance, the piston member may be constituted by a plate which floats on the liquid and which is movable and falls as the level of liquid in the cylinder drops. Typically, in this embodiment, a withdrawal tube is connected rigidly to the floating plate, with the withdrawal opening immersed in the liquid, so that the withdrawal point remains at a constant distance from the level of the liquid, in contact with the atmosphere. In this case, the pressure means referred to in the definition of the invention are constituted by the atmospheric pressure which constitutes an example of the application of a constant force. The flow is started by suction or by the creation of a partial vacuum in the transfer duct.
In the pressurized version, the piston member is slidable sealingly in contact with the walls of the cylinder which is thus sealed and is kept under pressure by means for exerting a constant pressure. Typically, these means comprise a weight which bears on the surface of the piston, or other means which can exert a constant force such as, for example, a mechanical or pneumatic device, for example, a stepper motor, or a source of reduced pressure, ensuring a constant pressure or a suction force. The term xe2x80x9cpressure meansxe2x80x9d is thus intended also to include means which exert a subatmospheric pressure.
Also in the pressurized version, in the preferred embodiment, the transfer means also comprise a withdrawal tube fixed to the piston and immersed in the liquid so that the withdrawal point remains at a constant distance from the liquid level. In this embodiment also, the flow in the transfer duct can be started by a suction effect or may be activated directly by the pressure exerted, or by a combination of the two.
In order to keep the difference in level between the withdrawal point and the point of delivery to the nozzle constant with variations in the liquid level, the transfer means may be constituted by a rigid siphon or may comprise a flexible tube, auxiliary rigid connecting means being provided for connecting the piston to the delivery nozzle; these embodiments will be described in greater detail with reference to the appended drawings.